Account for ?Sized type parameter bounds

src/librustc/traits/error_reporting/suggestions.rs

@@ -145,12 +145,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     let param_name = self_ty.to_string();
                     let constraint = trait_ref.print_only_trait_path().to_string();
                     if suggest_constraining_type_param(
+                        self.tcx,
                         generics,
                         &mut err,
                         &param_name,
                         &constraint,
                         self.tcx.sess.source_map(),
                         *span,
+                        Some(trait_ref.def_id()),
                     ) {
                         return;
                     }
@@ -1652,18 +1654,26 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
 /// Suggest restricting a type param with a new bound.
 pub fn suggest_constraining_type_param(
+    tcx: TyCtxt<'_>,
     generics: &hir::Generics<'_>,
     err: &mut DiagnosticBuilder<'_>,
     param_name: &str,
     constraint: &str,
     source_map: &SourceMap,
     span: Span,
+    def_id: Option<DefId>,
 ) -> bool {
     let restrict_msg = "consider further restricting this bound";
     if let Some(param) =
         generics.params.iter().filter(|p| p.name.ident().as_str() == param_name).next()
     {
-        if param_name.starts_with("impl ") {
+        if def_id == tcx.lang_items().sized_trait() {
+            // Type parameters are already `Sized` by default.
+            err.span_label(
+                param.span,
+                &format!("this type parameter needs to be `{}`", constraint),
+            );
+        } else if param_name.starts_with("impl ") {
             // `impl Trait` in argument:
             // `fn foo(x: impl Trait) {}` → `fn foo(t: impl Trait + Trait2) {}`
             err.span_suggestion(

src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs

@@ -217,12 +217,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                         tcx.hir().get_generics(tcx.closure_base_def_id(self.mir_def_id))
                     {
                         suggest_constraining_type_param(
+                            tcx,
                             generics,
                             &mut err,
                             &param.name.as_str(),
                             "Copy",
                             tcx.sess.source_map(),
                             span,
+                            None,
                         );
                     }
                 }

src/test/ui/consts/too_generic_eval_ice.stderr

@@ -18,7 +18,7 @@ LL | pub struct Foo<A, B>(A, B);
    | --------------------------- required by `Foo`
 LL | 
 LL | impl<A, B> Foo<A, B> {
-   |      - help: consider restricting this bound: `A: std::marker::Sized`
+   |      - this type parameter needs to be `std::marker::Sized`
 ...
 LL |         [5; Self::HOST_SIZE] == [6; 0]
    |             ^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
@@ -33,7 +33,7 @@ LL | pub struct Foo<A, B>(A, B);
    | --------------------------- required by `Foo`
 LL | 
 LL | impl<A, B> Foo<A, B> {
-   |         - help: consider restricting this bound: `B: std::marker::Sized`
+   |         - this type parameter needs to be `std::marker::Sized`
 ...
 LL |         [5; Self::HOST_SIZE] == [6; 0]
    |             ^^^^^^^^^^^^^^^ doesn't have a size known at compile-time

src/test/ui/dst/dst-object-from-unsized-type.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/dst-object-from-unsized-type.rs:8:23
    |
 LL | fn test1<T: ?Sized + Foo>(t: &T) {
-   |          -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |          - this type parameter needs to be `std::marker::Sized`
 LL |     let u: &dyn Foo = t;
    |                       ^ doesn't have a size known at compile-time
    |
@@ -14,7 +14,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/dst-object-from-unsized-type.rs:13:23
    |
 LL | fn test2<T: ?Sized + Foo>(t: &T) {
-   |          -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |          - this type parameter needs to be `std::marker::Sized`
 LL |     let v: &dyn Foo = t as &dyn Foo;
    |                       ^ doesn't have a size known at compile-time
    |

src/test/ui/issues/issue-27060-2.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/issue-27060-2.rs:3:5
    |
 LL | pub struct Bad<T: ?Sized> {
-   |                -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |                - this type parameter needs to be `std::marker::Sized`
 LL |     data: T,
    |     ^^^^^^^ doesn't have a size known at compile-time
    |

src/test/ui/traits/trait-suggest-where-clause.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `U` cannot be known at compilation tim
   --> $DIR/trait-suggest-where-clause.rs:11:20
    |
 LL | fn check<T: Iterator, U: ?Sized>() {
-   |                       -- help: consider further restricting this bound: `U: std::marker::Sized +`
+   |                       - this type parameter needs to be `std::marker::Sized`
 LL |     // suggest a where-clause, if needed
 LL |     mem::size_of::<U>();
    |                    ^ doesn't have a size known at compile-time
@@ -19,7 +19,7 @@ error[E0277]: the size for values of type `U` cannot be known at compilation tim
   --> $DIR/trait-suggest-where-clause.rs:14:5
    |
 LL | fn check<T: Iterator, U: ?Sized>() {
-   |                       -- help: consider further restricting this bound: `U: std::marker::Sized +`
+   |                       - this type parameter needs to be `std::marker::Sized`
 ...
 LL |     mem::size_of::<Misc<U>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time

src/test/ui/union/union-sized-field.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/union-sized-field.rs:4:5
    |
 LL | union Foo<T: ?Sized> {
-   |           -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |           - this type parameter needs to be `std::marker::Sized`
 LL |     value: T,
    |     ^^^^^^^^ doesn't have a size known at compile-time
    |
@@ -14,7 +14,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/union-sized-field.rs:9:5
    |
 LL | struct Foo2<T: ?Sized> {
-   |             -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |             - this type parameter needs to be `std::marker::Sized`
 LL |     value: T,
    |     ^^^^^^^^ doesn't have a size known at compile-time
    |
@@ -26,7 +26,7 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
   --> $DIR/union-sized-field.rs:15:11
    |
 LL | enum Foo3<T: ?Sized> {
-   |           -- help: consider further restricting this bound: `T: std::marker::Sized +`
+   |           - this type parameter needs to be `std::marker::Sized`
 LL |     Value(T),
    |           ^ doesn't have a size known at compile-time
    |

src/test/ui/unsized/unsized-bare-typaram.stderr

@@ -4,9 +4,9 @@ error[E0277]: the size for values of type `T` cannot be known at compilation tim
 LL | fn bar<T: Sized>() { }
    |    --- - required by this bound in `bar`
 LL | fn foo<T: ?Sized>() { bar::<T>() }
-   |        --                   ^ doesn't have a size known at compile-time
+   |        -                    ^ doesn't have a size known at compile-time
    |        |
-   |        help: consider further restricting this bound: `T: std::marker::Sized +`
+   |        this type parameter needs to be `std::marker::Sized`
    |
    = help: the trait `std::marker::Sized` is not implemented for `T`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

src/test/ui/unsized/unsized-enum.stderr

@@ -5,9 +5,9 @@ LL | enum Foo<U> { FooSome(U), FooNone }
    | ----------- required by `Foo`
 LL | fn foo1<T>() { not_sized::<Foo<T>>() } // Hunky dory.
 LL | fn foo2<T: ?Sized>() { not_sized::<Foo<T>>() }
-   |         --                         ^^^^^^ doesn't have a size known at compile-time
+   |         -                          ^^^^^^ doesn't have a size known at compile-time
    |         |
-   |         help: consider further restricting this bound: `T: std::marker::Sized +`
+   |         this type parameter needs to be `std::marker::Sized`
    |
    = help: the trait `std::marker::Sized` is not implemented for `T`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

src/test/ui/unsized/unsized-enum2.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `W` cannot be known at compilation tim
   --> $DIR/unsized-enum2.rs:23:8
    |
 LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
-   |        -- help: consider further restricting this bound: `W: std::marker::Sized +`
+   |        - this type parameter needs to be `std::marker::Sized`
 LL |     // parameter
 LL |     VA(W),
    |        ^ doesn't have a size known at compile-time
@@ -15,7 +15,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized-enum2.rs:25:8
    |
 LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
-   |                   -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |                   - this type parameter needs to be `std::marker::Sized`
 ...
 LL |     VB{x: X},
    |        ^^^^ doesn't have a size known at compile-time
@@ -28,7 +28,7 @@ error[E0277]: the size for values of type `Y` cannot be known at compilation tim
   --> $DIR/unsized-enum2.rs:27:15
    |
 LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
-   |                              -- help: consider further restricting this bound: `Y: std::marker::Sized +`
+   |                              - this type parameter needs to be `std::marker::Sized`
 ...
 LL |     VC(isize, Y),
    |               ^ doesn't have a size known at compile-time
@@ -41,7 +41,7 @@ error[E0277]: the size for values of type `Z` cannot be known at compilation tim
   --> $DIR/unsized-enum2.rs:29:18
    |
 LL | enum E<W: ?Sized, X: ?Sized, Y: ?Sized, Z: ?Sized> {
-   |                                         -- help: consider further restricting this bound: `Z: std::marker::Sized +`
+   |                                         - this type parameter needs to be `std::marker::Sized`
 ...
 LL |     VD{u: isize, x: Z},
    |                  ^^^^ doesn't have a size known at compile-time

src/test/ui/unsized/unsized-inherent-impl-self-type.stderr

@@ -5,9 +5,9 @@ LL | struct S5<Y>(Y);
    | ---------------- required by `S5`
 LL | 
 LL | impl<X: ?Sized> S5<X> {
-   |      --         ^^^^^ doesn't have a size known at compile-time
+   |      -          ^^^^^ doesn't have a size known at compile-time
    |      |
-   |      help: consider further restricting this bound: `X: std::marker::Sized +`
+   |      this type parameter needs to be `std::marker::Sized`
    |
    = help: the trait `std::marker::Sized` is not implemented for `X`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

src/test/ui/unsized/unsized-struct.stderr

@@ -5,9 +5,9 @@ LL | struct Foo<T> { data: T }
    | ------------- required by `Foo`
 LL | fn foo1<T>() { not_sized::<Foo<T>>() } // Hunky dory.
 LL | fn foo2<T: ?Sized>() { not_sized::<Foo<T>>() }
-   |         --                         ^^^^^^ doesn't have a size known at compile-time
+   |         -                          ^^^^^^ doesn't have a size known at compile-time
    |         |
-   |         help: consider further restricting this bound: `T: std::marker::Sized +`
+   |         this type parameter needs to be `std::marker::Sized`
    |
    = help: the trait `std::marker::Sized` is not implemented for `T`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
@@ -19,9 +19,9 @@ LL | fn is_sized<T:Sized>() { }
    |    -------- - required by this bound in `is_sized`
 ...
 LL | fn bar2<T: ?Sized>() { is_sized::<Bar<T>>() }
-   |         --             ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |         -              ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |         |
-   |         help: consider further restricting this bound: `T: std::marker::Sized +`
+   |         this type parameter needs to be `std::marker::Sized`
    |
    = help: within `Bar<T>`, the trait `std::marker::Sized` is not implemented for `T`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

src/test/ui/unsized/unsized-trait-impl-self-type.stderr

@@ -5,9 +5,9 @@ LL | struct S5<Y>(Y);
    | ---------------- required by `S5`
 LL | 
 LL | impl<X: ?Sized> T3<X> for S5<X> {
-   |      --         ^^^^^ doesn't have a size known at compile-time
+   |      -          ^^^^^ doesn't have a size known at compile-time
    |      |
-   |      help: consider further restricting this bound: `X: std::marker::Sized +`
+   |      this type parameter needs to be `std::marker::Sized`
    |
    = help: the trait `std::marker::Sized` is not implemented for `X`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

src/test/ui/unsized/unsized-trait-impl-trait-arg.stderr

@@ -2,9 +2,9 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized-trait-impl-trait-arg.rs:8:17
    |
 LL | impl<X: ?Sized> T2<X> for S4<X> {
-   |      --         ^^^^^ doesn't have a size known at compile-time
+   |      -          ^^^^^ doesn't have a size known at compile-time
    |      |
-   |      help: consider further restricting this bound: `X: std::marker::Sized +`
+   |      this type parameter needs to be `std::marker::Sized`
    |
    = help: the trait `std::marker::Sized` is not implemented for `X`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

src/test/ui/unsized3.stderr

@@ -1,42 +1,34 @@
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized3.rs:7:13
    |
+LL | fn f1<X: ?Sized>(x: &X) {
+   |       - this type parameter needs to be `std::marker::Sized`
 LL |     f2::<X>(x);
    |             ^ doesn't have a size known at compile-time
 ...
 LL | fn f2<X>(x: &X) {
-   |    -- - required by this bound in `f2`
+   |    -- -- help: consider relaxing the implicit `Sized` restriction: `: ?Sized`
+   |       |
+   |       required by this bound in `f2`
    |
    = help: the trait `std::marker::Sized` is not implemented for `X`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
-help: consider further restricting this bound
-   |
-LL | fn f1<X: std::marker::Sized +  ?Sized>(x: &X) {
-   |       ^^^^^^^^^^^^^^^^^^^^^^^
-help: consider relaxing the implicit `Sized` restriction
-   |
-LL | fn f2<X: ?Sized>(x: &X) {
-   |        ^^^^^^^^
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized3.rs:18:13
    |
+LL | fn f3<X: ?Sized + T>(x: &X) {
+   |       - this type parameter needs to be `std::marker::Sized`
 LL |     f4::<X>(x);
    |             ^ doesn't have a size known at compile-time
 ...
 LL | fn f4<X: T>(x: &X) {
-   |    -- - required by this bound in `f4`
+   |    -- -   - help: consider relaxing the implicit `Sized` restriction: `+  ?Sized`
+   |       |
+   |       required by this bound in `f4`
    |
    = help: the trait `std::marker::Sized` is not implemented for `X`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
-help: consider further restricting this bound
-   |
-LL | fn f3<X: std::marker::Sized +  ?Sized + T>(x: &X) {
-   |       ^^^^^^^^^^^^^^^^^^^^^^^
-help: consider relaxing the implicit `Sized` restriction
-   |
-LL | fn f4<X: T +  ?Sized>(x: &X) {
-   |            ^^^^^^^^^
 
 error[E0277]: the size for values of type `X` cannot be known at compilation time
   --> $DIR/unsized3.rs:33:8
@@ -45,7 +37,7 @@ LL | fn f5<Y>(x: &Y) {}
    |    -- - required by this bound in `f5`
 ...
 LL | fn f8<X: ?Sized>(x1: &S<X>, x2: &S<X>) {
-   |       -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |       - this type parameter needs to be `std::marker::Sized`
 LL |     f5(x1);
    |        ^^ doesn't have a size known at compile-time
    |
@@ -57,7 +49,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized3.rs:40:8
    |
 LL | fn f9<X: ?Sized>(x1: Box<S<X>>) {
-   |       -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |       - this type parameter needs to be `std::marker::Sized`
 LL |     f5(&(*x1, 34));
    |        ^^^^^^^^^^ doesn't have a size known at compile-time
    |
@@ -70,7 +62,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized3.rs:45:9
    |
 LL | fn f10<X: ?Sized>(x1: Box<S<X>>) {
-   |        -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |        - this type parameter needs to be `std::marker::Sized`
 LL |     f5(&(32, *x1));
    |         ^^^^^^^^^ doesn't have a size known at compile-time
    |
@@ -87,7 +79,7 @@ LL | fn f5<Y>(x: &Y) {}
    |    -- - required by this bound in `f5`
 ...
 LL | fn f10<X: ?Sized>(x1: Box<S<X>>) {
-   |        -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |        - this type parameter needs to be `std::marker::Sized`
 LL |     f5(&(32, *x1));
    |        ^^^^^^^^^^ doesn't have a size known at compile-time
    |

src/test/ui/unsized5.stderr

@@ -2,7 +2,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized5.rs:4:5
    |
 LL | struct S1<X: ?Sized> {
-   |           -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |           - this type parameter needs to be `std::marker::Sized`
 LL |     f1: X,
    |     ^^^^^ doesn't have a size known at compile-time
    |
@@ -14,7 +14,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized5.rs:10:5
    |
 LL | struct S2<X: ?Sized> {
-   |           -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |           - this type parameter needs to be `std::marker::Sized`
 LL |     f: isize,
 LL |     g: X,
    |     ^^^^ doesn't have a size known at compile-time
@@ -47,7 +47,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized5.rs:25:8
    |
 LL | enum E<X: ?Sized> {
-   |        -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |        - this type parameter needs to be `std::marker::Sized`
 LL |     V1(X, isize),
    |        ^ doesn't have a size known at compile-time
    |
@@ -59,7 +59,7 @@ error[E0277]: the size for values of type `X` cannot be known at compilation tim
   --> $DIR/unsized5.rs:29:8
    |
 LL | enum F<X: ?Sized> {
-   |        -- help: consider further restricting this bound: `X: std::marker::Sized +`
+   |        - this type parameter needs to be `std::marker::Sized`
 LL |     V2{f1: X, f: isize},
    |        ^^^^^ doesn't have a size known at compile-time
    |